A fire extinguisher can make a perfect high pressure air storage tank. It can be quite useful for refilling a pneumatic cannon for a second shot without having to fire up the air compressor or bicycle pump. Although due to its low volume (depending on size) it is not suitable for an automatic BB or airsoft type cannon as they have rather hefty air supply requirments. It also has many other uses around the home or workshop. For example; using it as a air duster or even putting in those few extra nails where the air compressor hose just can't reach. Most importantly it is relatively easy to prefrom the required modifications provided the induvidsual has the required tools and skills. The entire project is easy to complete in an afternoon.

I have attached basic MS paint drawing of the various modifications preformed. You should be able to figure out how to complete this from the drawing alone. The most difficult part of the procedure is adapting standard fittings to connect to the odd threads of the fire extinguisher, I overcame this problem but removing the threads both on the fire extinguisher socket and adaptor. This is easily achieved with a drill, file and a drill bit. After removing the threads the two pieces are carefully hammered together and then soldered. In the last picture you can see the final finished product.

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And the most beautiful part is: Fire extinguishers are so easy to come by! People leave them lying around everywhere!(That was a bad joke. No stealing fire extinguishers, people)

Actually, I think I do have a few old tanks around, and this might be pretty useful. A couple questions:Can you charge it thru the top? Or just in and out thru the same port?What pressure is it rated to?Are all fire extinguishers basically the same design? I don't know a lot about them, but I do know there are different types to use on different kinds of fires. (wood fires, chemical fires, electrical fires, etc.)

in regards to your first question, Absolutely! if you can find or machine and appropriate fitting that would allow you to refill from the top then there be be no problem. Although if it was up to me I would place a valve there to give an accurate reading of pressure. An easier solution though would be to obtain a "T" fitting, that would allow you to have an IN and OUT port if you so wish.

In response to pressure ratings, the most common type of fire extinguisher you would come across is a dry powder type. These fire extinguishers are extremely versatile and can be used on any fire. They are pressure rated to around the 300psi mark this one that I have has 21bar, approximately 304psi printed on the bottle. Other common types are water and carbon dioxide extinguishers. I do not those the exact pressure rating of these types but a Co2 extinguisher is going to have a rating magnitudes of times higher due to the immense pressure required to store a significant amount of compressed Co2. Although they are much heavier. I'm sure a Google search will reveal more exact specifications

Any fire extinguisher will be suitable for this task, many will have a tube protruding into the body of the cylinder but this is easily removed. In fact it does not even have to be removed if you so wish. Obviously you must discharge and clean the fire extinguisher before attempting modifications. The Dry Powder variety contain merely Sodium Bicarbonate (baking soda) or Potassium Bicarbonate, which is almost the same as baking soda. If you want to avoid a messy cleanup, simply depress the valve for one short burst. The powder will clog up the valve and the compressed are will slowly escape away. Then after you have checked to ensure it is completely empty of compressed air you can simply unscrew the top valve assembly and pour out the power into a plastic bag or two for easy disposal.

Also as a last note, please be aware that I am no professional in the field of fire extinguishers or high pressure containment. I have only brought my fire extinguisher up to a pressure of 75psi to test for leaks. Obviously the weakest point in the system is the soldered joint. I don't know at what pressure it will fail and really it depends on the quality of work preformed by the modifier. Although I am quite confident that it will hold fine at much higher pressures. The very worse that could happen is the joint coming apart and releasing the compressed air. But if you have the appropriate tapper would that would probably be a better solution.

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Last edited by Alessandro on Sat Oct 07, 2006 5:39 pm, edited 1 time in total.

Although I am quite confident that it will hold fine at much higher pressures.

Please be the guinea pig and let us know. I just hosted an annual extinuguisher training and was informed that a water extinguisher, which was the highest pressure of the few he had, was only 64psi. You make it sound as if there would be no big deal if there was a failure, but I'd be happy to show you what small objects launched at 75psi will do to common, everyday objects!

I have put a 5BC dry chem to 120 psi to use as a squirt gun.

Also be warned, if the used ext. you are using has a yellowish content, DO NOT BREATH IT. "Carbon Tetrachloride" extinguishers are being phased out, as the material has been found to be a severe carcinogen, causing fibrosis of the lungs.

Now, how many of these do I need to backup my cannon with twin 3" rezzies?

Although I am quite confident that it will hold fine at much higher pressures.

Even if the soldered joint did fail, it would be by no means spectacular. I'm sure everyone here has used "quick disconnect" sockets, well a similar thing would also happen in the event of failure of this joint. The reason for this is that the "barrel" would be only about a centimeter long as you can see in the diagram and so the parts only have an extremely short distance in which to be accelerated. The two parts would merely separate and fly back a few inches accompanied with a loud rush of air. 75psi means nothing without knowing all the factors involved.

I was not referring to the fine extinguisher itself which, as I mentioned earlier has a pressure rating of 304psi. Every gauge I've seen on a dry powder fire extinguisher has had a range of 0kpa-2000kpa (0-290psi). 64psi, even for a what is just basically a squirt gun seems low, are you sure that was the pressure rating or just the operating pressure? The operating pressure is about half of the pressure rating, this is to ensure that in the event of a fire when the extinguisher can be exposed to extreme heat, its pressure rating is not exceeded. Carbon dioxide fire extinguishers are tested to a massive, 1,250psi. <a href="http://erd.dli.mt.gov/safetyhealth/brochures/fireextinguishersafety.pdf#search=%22carbon%20dioxide%20fire%20extinguisher%20kpa%22">Here</a> is a reference, scroll down to page 8.

You mentioned that the fittings were soldered? You mean I can use rosin core lead to attach fittings with the same equipment used for electronics? Asking because I've not worked with metal before. I've got a spare aluminum fire extinguisher that looks ripe for modification.

Yep, same old 60/40 lead/tin that I use for electronics also. You won't be able to use a soldering iron though, you'll need to head it up with a torch. If you want something stronger than solder, you should use some sort of brazing alloy.

Edit: Actually, co2 fire extinguishers are rated to 1250psi, but seeing as most people don't have co2 regulators and it would be impossible and not to mention dangerous to perform these modifications with a compressed cylinder. It not a good idea.